An electro-photographic type image forming apparatus such as a laser printer is provided with a fixing device including a heating member and a pressure roller. The heating member is a fusing belt heated by a heater, and the pressure roller is configured to be rotated while contacting the heating member. The pressure roller has a roller portion whose diameter is changed by the expansion thereof due to increase in temperature. As a result, in the fixing device in which a sheet is conveyed upon input of driving force into the pressure roller, sheet conveying speed may be changed due to change in peripheral velocity of the pressure roller, which is caused by the change in the diameter of the pressure roller.
Japanese Patent Application publication No. 2007-298720 discloses a fixing device provided with a thermistor for detecting temperature of a cored bar of the pressure roller, so that a rotation speed of the pressure roller is controlled on a basis of the temperature detected by the thermistor.